Chromatin prepared from various tissues undergo proteolysis during chromatin dissociation, fractionation of chromosomal proteins and reconstitution of chromatin in the presence of salt and urea. The proteolysis appears to be due to a 25,000 dalton protease tightly bound to chromatin, and the degradation of chromosomal proteins can be blocked by 1 mM concentration of diisopropylfluorophosphate, phenylmethanesulfonylfluoride, and carbobenzoxyphenylalanyl chloromethylketone. The 25,000 dalton protease is present in all tissues and cells so far examined, but the enzyme is present in an inactive form in certain cells such as chicken erythrocytes and some cell lines grown in tissue culture. Work is in progress for purification of the 25,000 dalton protease and for the mechanism of inactivation of the enzyme in erythrocytes. BIBLIOGRAPHIC REFERENCES: Chae, C-B, Gadski, R.A., Carter, D.B. and Efird, P.H., Integrity of Proteins in Reconstituted Chromatin, Biochemical Biophysical Research Communications, 67, 1459-1465 (1975). Carter, D.B. and Chae, C-B., Chromatin-Bound Protease: Degradation of Chromosomal Proteins Under Chromatin Dissociation Conditions, Biochemistry, 15, 180-185 (1976).